TOS is the operating system designed for the Atari ST and TT series of computers. The Atari ST and TT use a Motorola 68k based CPU (The same CPU used by early Macintoshes) and was first manufactured in 1985.

The operating system consists of two parts.
TOS - "The Operating System". Kind of a customized version of CP/M provided by Digital Research.
GEM - "Graphics Environment Manager". The GUI shell that runs on top of TOS. This shares much in common with the DOS version of GEM.

Both TOS and GEM are stored entirely in the system's ROM.

This is the GEM desktop running on TOS displaying its desktop info screen.
The actual resolution of the default display is 320*200, obviously meant for viewing on a TV or composite monitor, but these screen shots have been enlarged to 640*400 for viewability. (Yet because the file size of the screen shots is so small I am not bothering with thumbnails for this page).
 

This is a blank desktop. Instead of disk icons, there are file drawers. The drives / drawers are assigned letters A, B, C and so on, similar to how CP/M and DOS organize drives.

Just imagine if IBM had gone with CP/M for the original IBM PC instead of Microsoft DOS. We might all be running Digital Research GEM 2004 right now.

The windows in TOS/GEM are Mac-like and can overlap and resize. None of the versions of GEM for Atari TOS were affected by the Apple lawsuit so all of the Atari versions retained these features.

Folders can be viewed as icons or by text and sorted by name, data, size, and type. This is the same as the DOS version of GEM.

Using the control box in the upper right of the folder windows, folders can be sized to take up the entire screen.
 

Like on GEM for DOS the trash icon is also not a folder.
 

The preferences allow you to select different video resolutions. This screen shot is in Medium resolution. High resolution is not available unless a special monochrome monitor attached.

An atari ST can run GEM in the following resolutions
"ST Low" -320*200*16 colors
"ST Medium" - 640*200*4 colors
"ST High" - 640*400*2 colors

An Atari TT can run GEM in higher resolutions:
"TT Low" - 320*480*16 colors
"TT Medium" - 640*480*16 colors
"TT High" - 1280*960*2 colors

As for shutting down the computer, there is no shutdown option in the GUI. You just turn off the power.

A screen shot of TOS 2.06 running in "medium" resolution. Nothing has radically changed in the UI in this version, but there are a few more options and settings that can be selected. There are also several other later versions of TOS as well including 3.x and 4.x versions.